Radiation receiver

ABSTRACT

The apparatus for collecting radiant energy and converting same to alternate energy form includes a housing having an interior space and a radiation transparent window allowing, for example, solar radiation to be received in the interior space of the housing. Means are provided for passing a stream of fluid past said window and for injecting radiation absorbent particles in said fluid stream. The particles absorb the radiation and because of their very large surface area, quickly release the heat to the surrounding fluid stream. The fluid stream particle mixture is heated until the particles vaporize. The fluid stream is then allowed to expand in, for example, a gas turbine to produce mechanical energy. In an aspect of the present invention properly sized particles need not be vaporized prior to the entrance of the fluid stream into the turbine, as the particles will not damage the turbine blades. In yet another aspect of the invention, conventional fuel injectors are provided to inject fuel into the fluid stream to maintain the proper temperature and pressure of the fluid stream should the source of radiant energy be interrupted. In yet another aspect of the invention, an apparatus is provided which includes means for providing a hot fluid stream having hot particles disbursed therein which can radiate energy, means for providing a cooler fluid stream having cooler particles disbursed therein, which particles can absorb radiant energy and means for passing the hot fluid stream adjacent the cooler fluid stream to warm the cooler fluid and cooler particles by the radiation from the hot fluid and hot particles.

BACKGROUND OF THE INVENTION

The invention described herein arose under work at Lawrence BerkeleyLaboratory in the course of, or under, contract W-7405-ENG-48 betweenthe U.S. Department of Energy (formerly Energy Research and DevelopmentAdministration) and the University of California.

RELATED APPLICATION

This is a division of U.S. patent application Ser. No. 061,165 filedJuly 26, 1979, now U.S. Pat. No. 4,313,304 issued Feb. 2, 1982.

The present invention relates to apparatus and method for collectingradiant energy and converting same into an alternate energy form and inparticular to collecting solar radiant energy by having same absorbed bysmall particles disbursed in a fluid stream.

BACKGROUND ART

Prior art radiant energy collectors can include fluid carrying channeldevices which are coated with, for example, a black, radiant energyabsorbent material. As the channel members conduct fluid therethrough,the radiant energy, as from the sun, absorbed by the black coating isre-radiated, conducted and convected to the moving fluid. In such anarrangement, the black absorbent covering is generally much hotter thanthe fluid being heated. As a consequence, the thermal efficiency of sucha system is not as great as would be desirable. Further, the channelmember is subject to thermal stress and subsequent breakdown due to thehigh temperatures developed and also due to cyclical heat-ups andcool-downs as, for example, happens during the non-operational and nightshutdown periods for such a solar system. Further, there is always thepossibility that hot spots can develop on the channel member resultingin the burning through of the channel member.

In actuality, a device for collecting solar radiation using the aboveprinciple generally requires a plurality of channel members, throughwhich fluid must be pumped, in order to provide enough surface areawhich is coated with the black absorbent material to adequately andthoroughly heat the fluid. Thus, an additional disadvantage of thissystem is that there are significant pumping losses from pumping thefluid through the various bends and other inherent restrictions. Also,such devices are generally quite heavy, and there is always thepossibility, as indicated above, that the hardware which connects theconduits will expand and contract at a rate different from the conduitsthemselves and thus eventually lead to leakage problems and amalfunction of the system.

In another prior device that is disclosed in U.S. Pat. No. 4,055,948,issued on Nov. 1, 1977 to Robert A. Kraus, a solar thermal radiationabsorption and conversion system includes multiple arrays ofsun-tracking mirror heliostats. The heliostats focus incoming thermalradiant energy through a highly light-transparent, radiation receiving,fluid containment shell located on top of a tall central tower. Aradiant, heat asbsorbing fluid passes through the containment shell andis heated. The fluid includes minute particles of colloidal sizesuspended in a transparent heat, transfer fluid. The fluid is channeledbetween a first highly light-transparent and a second highly lightreflecting wall of the shell. The minute particles, which are defined asbeing dull-black and non-reflecting, absorb the radiant energy flux andreadily heat the fluid stream in which they are suspended. The thusheated fluid is sent through a heat exchanger where heat is transferredto a secondary fluid for driving a turbine. The cooled fluid, includingthe particles, is returned to the containment shell for reheating. Inthis system, the primary fluid which includes the particles must beisolated by the secondary fluid so that the particles do not come incontact with the fast turning turbine blades and cause damage thereto.Further, in such a system, there is always the problem of uneven heatingor the development of hot spots which tend to shorten the life of thesystem by burning through the apparatus. Another problem associated withthis device is that the particles agglomerate, reducing their lightcollection and thermal efficiency and also can collect on or about thefirst highly light-transparent wall, reducing the radiation whichreaches the moving fluid with the colloidal suspension of particles.

SUMMARY OF THE INVENTION

The present invention is directed to overcoming one or more of theproblems as set forth above.

In one aspect of this invention a method for converting a radiant energyflux to an alternate form of energy includes the steps of disbursingradiant energy absorbent particles in a fluid stream, passing the fluidstream and the disbursed particles past the radiant energy flux to heatthe particles and the fluid, allowing the particles to vaporize in theradiant energy flux, and allowing the heated moving stream to expand toproduce the alternate energy form.

In another aspect of the invention, an apparatus is provided forcarrying out the above method.

The advantages over the prior art of this method and apparatus are asfollows. First as the particles can absorb radiant energy, and as theyhave a large surface area in comparison to their volume, they tend toheat quickly and transfer their heat to the surrounding fluid stream.Accordingly, the temperature difference between the particles and thefluid stream is small, accounting for the high thermal efficiency of thesystem. Further, unlike the prior art, the internal and/or externalsurface of the housing or channel which conducts the fluid stream is notcovered with a radiation absorbent material, in fact the internalsurplus of the housing may be covered with a radiation reflectantmaterial to reflect radiation into the stream of fluid and disbursedparticles. Consequently, the highest temperature in the apparatus of theinvention occurs in the gas itself and not in the housing which containsand channelizes the gas. In the prior art, the highest temperatureoccurs on the channel housing which accounted for thermal stress andfatigue in said housing. Stated alternatively, in the present inventionthe temperature of the housing for any given gas temperature is lowerthan the temperature of the housing of conventional systems.

Further, any hot spots are eliminated by the vaporization of the energyabsorbent particles and the subsequent replacement of the vaporizedparticles by other particles moving in the fluid stream. Eventually allthe particles are vaporized before they are allowed to expand in, forexample, a gas turbine to produce an alternate form of energy, which inthis case is mechanical energy. This being the case, there are noparticles remaining in the fluid stream to damage the fast movingturbine blades. Thus, there is no need for a primary and secondary fluidstream and an inefficient heat exchanger coupling said streams totransfer heat to the secondary gas and allow said secondary gas toexpand. Also, as the particles vaporize, there is no need to condensesaid particles out before the gas goes to the turbine as might berequired in a one loop system. Further, due to vaporization, there is noagglomeration of the particles. In the present invention, thevaporization of the particles does not create an emissions problem asthe waste carbon dioxide is minimal.

Further, the apparatus of the invention is lightweight as there is noneed for a plurality of conduits or channels as in prior devices andaccordingly there is no significant pumping loss.

The disbursed particles provide for high optical efficiency with lowscattering so that a minimal amount of radiation is reflected back outof the housing.

In another aspect of the invention, conventional fuel injection meansand combustion means are provided in the housing to maintain the gastemperature should there be an interruption in the radiation received.Such a conventional system can be conveniently incorporated in thesystem at minimal additional cost due to the simplicity of the systemdesign. Further, this eliminates any need for highly expensive andlow-efficiency heat storage systems to compensate for such loss ofradiation flux.

In another aspect of the invention, the particles are submicron indiameter and thus do not have to be vaporized as they can pass throughthe gas turbine without damaging the high speed turbine blades.

In another aspect of the invention, a radiation receiver comprises ahousing having an interior space, a radiation transparent window forallowing radiation to be received in the interior space of the housing,and a radiation transparent heat shield. The heat shield is spaced fromthe radiation transparent window and divides the interior space into afirst compartment defined between the window and the heat shield and asecond compartment. Means are provided for passing a stream of fluidthrough the first and second compartments. Means are provided forinjecting radiation absorbent particles into the stream of fluid passingthrough said second compartment and means are provided for preventingradiation absorbent particles from passing through the firstcompartment. Thus, there can be no agglomeration, as in the prior art,of particles on the radiation transparent window to effect the thermalefficiency of the system. Further, any particles agglomerated thereonwould soon be vaporized.

In yet another aspect of the invention, the heat shield can be removedwhen lower operating temperatures do not require same.

In another aspect of the invention, a method of transferring energyincludes the steps of providing a hot fluid system having hot particlesdisbursed therein, which particles can radiate energy and the step ofproviding a cooler fluid stream having cooler particles disbursedtherein, which particles can absorb radiant energy. The method furtherincludes passing the hot fluid stream adjacent the cooler fluid streamto warm the cooler fluid and cooler particles by radiation from the hotparticles. Such a heat transfer system has all the advantages of theprevious embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the system of the invention which includesa radiation receiver of the invention;

FIG. 2 is a perspective view of the embodiment of the radiation receiverof the invention;

FIG. 3 is a second embodiment of the radiation receiver of theinvention;

FIG. 4 is a cross-sectional view taken longitudinally through line 4--4of FIG. 3; and

FIG. 5 depicts an embodiment of a heat exchanger of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the figures, and in particular to FIG. 1, the radiantenergy collection and conversion apparatus is depicted and generallydenoted by the numeral 10. The schematic drawing in FIG. 1 represents anopen Brayton cycle. However, it is to be understood that a closedBrayton cycle or other thermodynamic cycle could be used. In the openBrayton cycle gas, such as air, enters inlet 12 and is compressed bycompressor 14. A conduit 18 directs the compressed gas to a recuperatoror preheater 20 where the compressed gas is preheated by the exhaust ofapparatus 10 which exits through exhaust or outlet 22. The preheated gasis delivered through conduit 24 to a caldron or radiation receiver 26.As will be discussed in greater detail hereinbelow, caldron 26 includesa radiation transparent window 28 which admits solar flux indicated byarrows 30 into receiver 26. Communicating with the upstream end ofreceiver 26 is a particle generator 32. Particle generator 32 produces avery small mass of fine, radiation absorbent particles, which in apreferred embodiment have a diameter in the submicron range, and injectssaid particles into the compressed gas stream prior to the gas streamflowing past the window 28. Details of the particle generator 32 and theparticles themselves will be discussed hereinbelow. In the reservoir 26the particles absorb the solar radiation flux and heat the gas. Sincethe particles are such efficient heat exchangers, their temperature doesnot rise substantially above the temperature of the compressed gas. Thegas-particle mixture continues to heat until the particles vaporize. Theheated gas then passes through conduit 34 and into expansion turbine 16which provides the power for operating compressor 14 and generator 36.The exhaust gas from turbine 16 is then directed by conduit 38 torecuperator 20 where the exhaust gas preheats the incoming compressedgas.

Also in a preferred embodiment a fuel injector and combustor means 40 isprovided in communication with conduit 34 between receiver 26 andturbine 16. Alternatively, fuel injector and combustor means 40 can beprovided in communication with conduit 24 between preheater 20 andreceiver 26. The fuel injection and combustor means 40 providesconventional means for heating the incoming compressed gas should thesolar flux fail for any reason. It is to be understood that such acombustor means is economically installed and used and eliminates thenecessity of expensive energy storage means which would otherwise benecessary to guarantee continued performance of the system during theperiods of little or no solar flux.

In a system design, the compressor, particle generator, turbine, andreceiver are located in a central upstanding tower (not shown), such asthat shown in U.S. Pat. No. 4,095,118, issued on June 13, 1978, toKenneth R. Rathbun. It is to be understood that in such configurationthe conventional fuel injection and combustor means can be located inthe central tower or at the base thereof, wherever such location isconvenient. The central tower is surrounded by an array of pivotallymounted reflectors disposed circumferentially thereabout, which reflectearth incident solar radiation to the receiver.

In the above open Brayton cycle, the exhaust includes a minimal amountof carbon dioxide due to the vaporization of the small mass of absorbentparticles. Certainly these levels of pollution would not violate anyexisting or future projected air quality control standards. The openBrayton cycle does not require large cooling towers and the attendantuse of water, which is usually associated with electric power plants.This being the case, it is possible to locate such systems in an ariddesert area where water is at a premium, but where consistent solar fluxis guaranteed. If a closed Brayton cycle were to be used, a workingfluid, such as water, would have to be recirculated through the system.This working fluid would be used to cool the gas before it isrecompressed, and would require the construction of large cooling towerfacilities to in turn cool the working fluid.

PARTICLE AND PARTICLE GENERATOR

The most appropriate method for producing particles for this apparatusinclude high intensity arcs, thermal decomposition of hydrocarbons orcarbon monoxide, and high temperature pyrolysis of organic resins. Thehigh intensity arc has been established as a method for industrialproduction of submicron particles. Carbon, metals, semi-conductors andmany other oxides have been produced at rates of kilograms to tens ofkilograms per hour using a single arc. Carbon blacks have been producedchemically by thermal decomposition by a quenching procedure known asthe channel process since 1872. The properties of these blacks varywidely, but they tend to be symmetrical in shape and to have particlediameters ranging from 0.01 to 0.5 micrometers. High temperaturepyrolysis is used to produce vitrous carbon that has extremely highdecomposition temperatures in air. Further discussion of these threemethods can be found in the following sources. The high intensity arcsare discussed by J. D. Holmgren, et al at p 129 of "Ultrafine Particles"(1963) W. E. Kuhn Ed., John Wiley & Sons, Inc., New York. The thermaldecomposition of hydrocarbons is discussed by P. L. Walker, Jr., p. 297"Ultrafine Particles" (1963) W. E. Kuhn Ed., John Wiley & Sons, Inc.,New York. And the high temperature pyrolysis is discussed by J. S.Nadeau in the 57th volume of the Journal of the American Ceramic Societyon pp 303 to 306.

It is to be understood that the disbursement of premanufactured powdersis extremely difficult due to the tendency of these small particles toagglomerate. Such disbursement must overcome the large surface forcesthat hold the particles together. Thus, it has been found that the bestway to produce particles is to generate them at the site, by one of theabove indicated methods, and entrain them in a gas stream, and conductthem to the receiver, thus minimizing the changes of agglomeration.

The operating temperature of the receiver 26 is determined by theoxidation rate of particles. Carbon is an ideal material because the gasreaction rates for various allotropes of carbon can vary over manyorders of magnitude and thus the appropriate temperature can be selectedby the use of an appropriate carbon. The use of carbon has theadditional feature that the combustion product is carbon dioxide. Itshould be emphasized at this point that the mass of particles used isextremely small compared to the amount of fuel burned by a conventionalfossil fuel power plant. Thus, the amount of carbon dioxide generated isless than one-one-hundredth of the production of a fossil fuel plant ofthe same power.

Particles distributed throughout a volume of gas is a very efficientabsorber of sunlight if the particle size and optical constants arechosen properly. If the characteristic absorption length of the lightpassing through the gases comprising the particles is greater than theparticle diameter, then the entire volume of the particle is active asthe absorber. For this and other reasons, submicron particles are used.The result is a high absorption coefficient for the incoming sunlight,or equivalently a high optical efficiency for the receiver. Since theinfrared re-radiation from the heated gas will be inhibited from leavingthe chamber by the window, further details of said window to bediscussed hereinbelow, the receiver will have a high overall efficiency.One consequence of this is that the receiver is not restricted to acavity type, but may be illuminated from all sides, as shown in theembodiments of FIGS. 3 and 4. This is an illustration of thecharacteristics of a diffuse suspension of small particles acting like ablack body absorber.

The combination of the large surface area and the small size ofparticles insures that the particle temperature stays to within afraction of the degree of the gas temperature. Thus, the highesttemperature present in the receiver is essentially that of the gas. Thisresults in much lower radiant temperatures in the chamber than for othersolar receivers which produce gases of the same temperature. This isparticularly true of solar receivers which have a surface thereof coatedwith a black, absorbent material and thus rely on re-radiationconduction and connection to heat the moving fluid.

It is to be understood that light passing through a medium containingsmall particles may be scattered or absorbed. If the particles aresufficiently small and are composed of material that is intrinsicallyabsorbing, the extinction (the name given to the combined effect ofscattering and absorption) of a beam of light passing through a mediumwill be dominated by absorption.

Some reflective losses can be expected due to reflection from thesurface of window 28. If quartz were used as the material for thewindow, with an index of refraction of 1.5, there would be a reflectionof 4% per surface area for normal solar radiation incidence. Thisreflectivity would be substantially reduced by the use ofanti-reflection coatings or by controlling the surface morphology.

Further, it is to be understood that in the submicron range, theabsorption rate is constant for the various small particle sizes. Thisis due to the fact that the entire mass of the particle is used as anabsorber, not merely the outer surface thereof.

FIRST PREFERRED EMBODIMENT OF SOLAR RECEIVER

FIG. 2 shows a more detailed perspective view of the structure of theradiation receiver 26. Receiver 26 includes the aforementioned concavewindow 28 which provides an opening in cylindrical housing 42. Housing42 defines an internal cavity 44 which contains a heat shield 46. Heatshield 46 is located adjacent the window 28. Heat shield 46 divides theinternal cavity 44 into a first member 48 which is defined between heatshield 46 and window 28 and a second chamber 50 which is defined betweenheat shield 46 and the remainder of housing 42. Mounted around the upperedge of housing 42 are a plurality of particle injectors 52 whichcommunicate with the particle generator 32.

The function of the window 28 of the receiver 26 is to allow the solarflux to enter the internal cavity 44, to confine the pressurizedgas-particle mixture and to prevent substantial loss of heat by infraredradiation. There are several materials that meet the solar transparencyrequirements for such a system. These include in a preferred embodimentPyrex® and quartz. These materials pass nearly the entire solar spectrumand are also opaque to radiation with wavelengths greater than fourmicrometers. This opacity in the infrared will reduce heat loss byradiation to a very low level.

The pressure requirements on the window are modest, on the order of fourto six atmospheres for an open cycle Brayton system. As Pyrex® hassuperior strength qualities and is cheaper than quartz, but does nothave the high temperature capability of quartz, the window can becomprised of Pyrex® to withstand the necessary pressures, while the heatshield can be comprised of quartz to withstand the necessarytemperatures. As can be seen in FIG. 2, the window 28 is concavedinwardly to insure that the window remains in compression, providing themaximum strength for the Pyrex®. Gas is circulated between the window 28and heat shield 46 to cool the window. The main flow of gas with theentrained particles flows through the second chamber 50. A secondarygaseous flow which does not contain particles can be directedcountercurrent to the particle flow through the first chamber 48 to coolthe window 28 and to insure that particles do not collect on said window28. The counterflow through the first chamber then mixes with theparticle laden flow at the upper portion of the receiver 26. It is to beunderstood that alternatively, the particle injectors 52 can be locatedmore downstream in the receiver 26 than is indicated in FIG. 2. If thisis the case, the flow through the first chamber 48 can be in the samedirection as the flow in chamber 50, as there will be little chance thatparticles will flow in first chamber 48. Further, it is to be understoodthat particle buildup on the window is not likely to be a seriousproblem, as the particles are so small that they move with the gasstream. If the particles do reach the window and adhere or do flowthrough chamber 48, they will quickly be heated to vaporization orcombustion temperatures since they are in the region of maximum solarradiation flux.

A reflective surface 54 reflects solar radiation back into the flow offluid.

Further, it is to be noted that the above receiver 28 does not require ahigh temperature pressure seal for the heat shield 46, as the pressureseal is provided by the window 28.

Also, it is to be noted that with the appropriate selection of operationparameters, the heat shield 46 of the receiver 26 can be eliminated, thewindow 28 then providing the necessary pressure sealing and temperatureshielding.

SECOND RADIATION RECEIVER EMBODIMENT

FIGS. 3 and 4 depict a second radiation receiver embodiment which may beused in place of receiver 26 and which is designated by the number 60.Receiver 60 includes a cylindrical housing 62 and a plurality of concavewindows 64 which allow housing 62 to be open on all sides. Concavewindows 64 are provided with temperature and pressure seals 66 at theirline of intersection. In a preferred embodiment, the window can becomprised of Pyrex®. In this embodiment, a heat shield 68 is provided inthe shape of a hexagon, the six flat surfaces thereof being associatedwith the concave surfaces of the window. A pillar is provided in thehousing for support purposes and for providing a reflective surface 69to reflect any solar radiation which strikes said surface back into theflow of fluid. Essentially all of the variations which are associatedwith the first embodiment of the receiver 28 discussed in conjunctionwith FIG. 2 can be applied to the receiver 60 of FIGS. 3 and 4.

The operation of the apparatus 10 is as described hereinabove. Further,it should be understood that with appropriately sized submicronparticles, said particles need not be vaporized as they will passthrough the turbine blades without causing damage thereto. Certainly, ifthe particles have diameters which are on the order of several microns,such particles would damage the turbine blade. Fortunately, withparticles in each submicron size range, the particles are simply sosmall that they follow the motion of the gas and avoid hitting theturbine blades.

Thus, it can be realized that there are many advantages in using theabove invention over the existing prior art. Since the receiverbasically consists of a hollow chamber with a window, there is no needfor heavy and complex heat exchanger elements, as is necessary with theprior art. Further, there is no pumping loss due to the forcing of gasesthrough pipes and small orifices. This has the effect of considerablyreducing the amount of energy required to overcome pressure losses.Further, in the above system, the particles can be vaporized so thatthere are no problems associated with maintenance, failures, heat stressor corrosion encountered with conventional solar heating systems. Also,there is no problem with thermo-decomposition of the working fluid orthe reflective coatings. Further, there is no high stagnationtemperature which can severely damage the collector system as theparticles and fluid itself maintains a temperature which is greater thanthe surrounding receiver.

SMALL PARTICLE GAS-TO-GAS HEAT EXCHANGER

A non-solar use of the above invention can be in a gas-to-gas heatexchanger. The present gas-to-gas heat exchangers have limited usebecause they generally have significant temperature drops. Thus, theseheat exchangers can usually be advantageously used only where largetemperature changes are tolerable or desired. Small temperature drops insuch systems can only be obtained through the use of very largesurfaces. However, it is noted that small particles suspended in a gasstream have a large surface area for the small volume of material used.

FIG. 5 illustrates a small particle gas-to-gas heat exchanger 80. Heatexchanger 80 includes a drum housing 82 which has an inlet 84 forreceiving a mixture of hot gases and particles and an outlet 86 forexhausting the hot mixture of gas and particles. Transparent tubes 88and 90 are positioned in housing 82 to provide channels for the flow ofa cooler mixture of gases and particles. The hot mixture of gases andparticles flows over transparent tubes 88 and 90 but does not mix withthe cooler air particle mixture which is conducted through said tubes 88and 90. The tubes 88 and 90 are transparent to radiation of a wavelength corresponding to the black body peak at the desired operatingtemperature.

The effectiveness of this gas-to-gas heat exchanger depends on thedesired operating temperature. Since the heat exchanger depends onradiative transfer, the temperature must be high enough for asubstantial amount of heat energy to be in that form. At temperatures of1500° K., roughly one-tenth of the energy acquired by carbon particlesat six atmospheres pressure is given up in the form of radiation. Sincethe gases by themselves are generally not good radiators at thesetemperatures, the particles would dominate the radiative transferprocess. At higher temperatures and lower pressures, the particles wouldbe even more effective at radiating and heating the gas-particlemicture. A use of the above invention would be with modern fluidized bedcombustor units utilizing Brayton cycles. These units presently must useexpensive, high temperature filtration to remove the particles from thegas stream before passing it through the turbine. The combustion processmust be carried out at a high pressure which for a large fluidized bedmeans that the vessels become very heavy. An efficient gas-to-gas heatexchanger would allow lower pressures that would simplify the design andeliminate the need for the high temperature filtration.

Other aspects, objects, and advantages of this invention can be obtainedfrom a study of the drawings, the disclosure, and the appended claims.

I claim:
 1. A radiation receiver comprising:a housing having an interiorspace; at least one radiation transparent window provided in theperiphery of said housing for allowing radiation to be received in theinterior space of said housing; a radiation transparent heat shieldspaced from said radiation transparent window wherein said heat shielddivides said interior space into a first compartment defined betweensaid window and said heat shield and a second compartment; means forpassing a first stream of fluid through said first compartment and asecond stream of fluid through said second compartment; and means forinjecting solar radiation absorbent vaporizable particles into thestream of fluid passing through said second compartment, wherebyagglomeration of the particles is eliminated and partical damage toassociated turbine blades is prevented.
 2. The apparatus of claim 1wherein said housing is provided with a single concave radiationtransparent window, and the means for passing a stream of fluid includesmeans for pressurizing said fluid.
 3. The apparatus of claim 1 whereinthe injecting means includes means for producing solar radiationvaporizable particles which are submicon in diameter.
 4. The apparatusof claim 1 including a radiation reflecting wall spaced from said heatshield.
 5. The apparatus of claim 1 wherein said first stream of fluidflows countercurrently to said second stream of fluid.
 6. A radiationreceiver comprising:a housing having an interior space; at least oneradiation transparent window provided in the periphery of said housingfor allowing radiation to be received in the interior space of saidhousing; a radiation transparent heat shield spaced from said radiationtransparent window wherein said heat shield divides said interior spaceinto a first compartment defined between said window and said heatshield and a second compartment; means for passing a first stream offluid through said first compartment and a second stream of fluidthrough said second compartment; means for injecting radiation absorbentparticles into the stream of fluid passing through said secondcompartment; and means for injecting combustible fuel in the fluidstream which passes through the second compartment.
 7. The apparatus ofclaim 6 including means for combusting said fuel.
 8. A radiationreceiver comprising:a generally cylindrical housing having an interiorspace; a plurality of concave radiation transparent windows providedabout the cylindrical periphery of said housing for allowing radiationto be received in the interior space of said housing; a radiationtransparent heat shield spaced from said radiation transparent windowwherein said heat shield divides said interior space into a firstcompartment defined between said window and said heat shield and asecond compartment contained within said heat shield; means for passinga first stream of fluid through said first compartment and a secondstream of fluid through said second compartment; and means for injectingradiation absorbent vaporizable particles into the stream of fluidpassing through said second compartment, whereby agglomeration of theparticles is eliminated and particle damage to associated equipmentthrough which the second stream of fluid passes is prevented.
 9. Theapparatus of claim 8 including means positioned in the secondcompartment for reflecting radiation toward said window.